1904.] PACKARD— ORIGIN OF MARKINGS OF ORGANISMS. 419 



what they can safely devour and what they must avoid. But if each 

 single bird has to learn this distinction by experience, a certain 

 number of distasteful butterflies must also fall victims to the inex- 

 perience of the young enemies. Now if two distasteful species are 

 sufficiently alike to be mistaken for one another, the experience ac- 

 quired at the expense of one of them will likewise benefit the other ; 

 both species together will only have to contribute the same number of 

 victims which each of them would have to furnish if they were dif- 

 ferent. If both species are equally common, then both will derive 

 the same benefit from their resemblance — each will save half the 

 number of victims which it has to furnish to the inexperience of its 

 foes. But if one species is commoner than the other, then the 

 benefit is unequally divided, and the proportional advantage for 

 each of the two species which arises from their resemblance is as 

 the square of their relative numbers. 



'' If two species are concerned, of which the one is very common 

 and the other very rare, then the advantage falls almost entirely on 

 the rarer species. If, for example, Acrcea thalia were a thousand 

 times commoner than Eueides parana, the latter would derive a 

 million times greater benefit from the resemblance of the two spe- 

 cies, whilst for the Acraea the benefit is practically ;///. Thus 

 Eueides pai'ana might by natural selection be converted into one 

 of the most exact mimics of Acrcea thalia, although it is just as dis- 

 tasteful as the species imitated. 



" On the other hand, if two or even several distasteful species are 

 about equally common, resemblance brings them a nearly equal 

 advantage, and each step which the other takes in this direction is 

 preserved by natural selection — they would always meet each other 

 numerically — so that finally one would not be able to say which of 

 them has served as the model for the others. In this manner are 

 explained those cases where several allied distasteful species {e.g., 

 Colcenis Julia, Eueides aliphera, and Dione Juno) resemble one 

 another — cases where such resemblance cannot be regarded as 

 inherited, and yet where neither of the species appear to claim to 

 have served as a model for the others. 



" To this category Ituna and Thyridia may belong, although the 

 first has probably made the greater step in passing from the former 

 dissimilarity to the present resemblance of the two species." 



It will be interesting to read the comments on this paper made 

 by Mr. Bates at the same meeting. He remarked that " he could 



